1. Field of the Invention
The present invention relates to semiconductor devices formed in wide band gap materials, and in particular to fabricating low resistance tunnel junctions in wide band gap materials.
2. Description of the Related Art
Junctions between differently doped regions of a single semiconductor material are called homojunctions. These junctions are formed, for example, when a single material abruptly transitions from one type of doping to another. An abrupt transition is typically one where a value of merit (in this instance doping concentration) changes by more than one percent per monolayer.
A p-n homojunction consists of a p-type and an n-type section of the same semiconductor materials in metallurgical contact with each other. When n- and p-type semiconductors are brought into contact to form a junction, electrons and holes diffuse from areas of high concentration towards areas of low concentration. Thus, electrons diffuse away from the n-region into the p-region, leaving behind positively charged ionized donor atoms in the n-region. In the p-region, the electrons recombine with abundant holes. Similarly, holes diffuse away from the p-region, leaving behind negatively charged ionized acceptor atoms. In the n-region, the holes recombine with abundant mobile electrons. As a result of this diffusion, a narrow region on both sides of the contact becomes almost totally depleted of mobile charged carriers. This region is called the depletion layer. The thickness of the depletion layer in the junction is inversely proportional to the concentration of dopants in the region.
A p-n homojunction typically will act as adiode. A particular type of degenerately doped p-n homojunction forms a conventional tunnel diode. Tunnel diodes, first discovered by L. Esaki, are described in the Physics of Semiconductors 2d Ed, Sze, Wiley Interscience Publisher, 1981, Chapter 9, pages 513-536. Esaki observed, while studying degenerately doped germanium p-n junctions, an anomalous current voltage characteristic in the forward biased direction of the junction. Esaki explained this characteristic as being the result of quantum tunneling across the depletion region. The width of the depletion region was therefore called a tunnel distance, or tunnel width. Esaki observed, under slight bias, charge carriers tunneling across the depletion region of what would be an impenetrable p-n junction at that bias without the quantum tunneling.
A bias applied across a tunnel diode is required to sustain a net tunneling current and one may define the tunneling resistance as the bias divided by current. Under certain conditions, the tunneling resistance can be low enough that the tunnel diode current-voltage relationship is essentially ohmic (linear). Three primary factors determine the tunneling resistance: the density of free electrons on one side of the junction, the density of holes on the other side, and the tunneling probability. The higher the value of these parameters, the lower the tunneling resistance. While it is generally a complex function of the details of the tunnel junction, the tunneling probability decreases roughly exponentially with tunneling distance. Thus, tunneling resistance is reduced when the tunnel width is as small as possible.
The simplest tunnel diode comprises a p-n homojunction in which both p and n sides are uniformly and degenerately doped. The depletion region or tunnel width is inversely proportional to the square root of the charge carrier density (the number of charge carriers per cubic centimeter) of the materials used to form the junction, and directly proportional to the size of the material's band gap.
Degenerative doping of the materials that form the tunnel junction reduces the tunnel width across which the charge carriers need to tunnel. Unfortunately, there is an upper limit to how much this mechanism can reduce the tunnel width. All dopants eventually reach a saturation solubility limit at which the material is no longer capable of absorbing further dopants without changing its composition. Once this saturation limit is reached, doping loses its ability to reduce the tunnel width. Furthermore, as the charge density increases the dopant ionization probability decreases according to basic semiconductor statistics, again limiting the ability of doping to reduce the tunnel width.
Homojunction tunnel junctions may be fabricated in periodic table group III-nitride semiconductor materials. Such materials include, but are not limited to, indium nitride, gallium nitride and aluminum nitride, and combinations thereof. One difficulty with these nitride materials is that their band gap is significantly larger than the band gap of other III-V materials. For example, gallium nitride (a III-nitride semiconductor material) has a band gap of roughly 3.4 electron volts (eV), while gallium arsenide (a conventional III-V semiconductor material) has a band gap of approximately 1.4 electron volts. This band gap difference is significant, because a larger band gap results in a larger, or wider, tunnel width. A tunnel junction with low tunneling resistance is very difficult to form in wide band gap materials such as gallium nitride or silicon carbide.